Aside from techniques for dipping a pipe, tube or conduit into a bath of a molten or solvent-liquefied synthetic-resin and removing the coated body therefrom, several methods have been suggested for the coating of such objects with thermoplastic synthetic-resin materials. For example, it has been suggested to fill a tube or pipe with synthetic-resin powder and then to heat the metal pipe with one or more induction coils which are disposed along the exterior of the pipe or are displaced relative thereto. After an initial fusion of the particles along the inner wall of the tube or pipe, nonadherent particles are removed from the tube and the latter is again heated by induction to cause the synthetic-resin to flow or coalesce in a more or less continuous layer along the interior of the pipe. This system has the disadvantage that the filling of the pipe with synthetic resin must take place with the pipe in an erect position, thereby eliminating the use of the method for extremely long or relatively narrow pipes. Furthermore, the deposition of synthetic resin along the wall of the pipe is nonuniform and the reflowing step often yields a nonuniform product. There are even adhesion problems in many cases. Finally, it should be mentioned that the apparatus required for the internal coating of pipe or tubes in this manner is relatively complex and expensive, and it is difficult often to remove the particles packed therein.